Pencil lead



Patented Feb. 24, 1942 2,273,935 PENCIL LEAD Isidor Chesler, Oceanport, N. J., assignor to Eagle Pencil Company, Inc., New York, N. Y., a corporation of Delaware No Drawing. Application September 9, 1940, Serial No. 356,031

13 Claims.

of rating or classification.

Inasmuch as the mark made by the specialized lead is not distinguishable by the eye from a mark made by an ordinary lead, and since such ordinary lead would not give the desired registration on the statistical machine, it is important to provide means for readily checking that the paper or card was in fact marked by the specialized lead. Such means is disclosed and claimed in my prior patent application Serial No. 311,944 filed December 30, 1039.

In practice, it has been found that where the rhodamine and other dyes mentioned in said prior application are incorporated in an electrolyte, which latter imparts the high conductivity required, sharp increase in resistance and loss in efiicacy for the desired purpose may be incurred under adverse climatic conditions as for instance in a very dry and hot climate. For in that environment, the electrolyte may dry out in whole or in part and the indicator dye may crystallize with increase of resistance of the mark, so that the same will not be reliably recorded by the statistical machine.

It is accordingly an object of the invention to provide a lead incorporating an indicator of the type referred to, the cost of production of which is not excessive, which is not subject to deterioration in any respect, even though used long after it has been fabricated, and the mark made by which lead will maintain its high electric conductivity under all conditions of use regardless of the ambient temperature or humidity.

Another object is to provide a lead of the above type, the electric conductivity of whose mark is predetermined, to be suificiently high to effect the recording, but not so high as to effect an unwanted recording of an inadvertent smudge spread for instance by a wet finger, to an adjoining space.

Another object is to provide a pencil lead useful for the foregoing purpose, which will have all of the' desirable properties of known leads of good quality, including smooth writing of predetermined softness or hardness, durability and uniformity, and which diners in practical performance from known leads of corresponding quality substantially only in that the electrical resistance of the mark is within the limits required by the construction and mode of operation of the statistical machine by which the markings are recorded.

Another object is to provide a lead of the above type, which, despite its high electrical conductivity, has no ingredients therein apt to destroy or impair the card or other sheet upon which the pencil is used, and which has no toxic ingredient apt to cause injury or discomfort by contact with the fingers or with the tongue of the user.

While the rhodamine dye solution or equivalent solution of eosine, methylene blue, methyl violet or the like referred to in the prior application has a tendency to crystallize with sharp increase in electric resistance of the mark where an electrolyte is used as the carrier, I have found that by the use of fatty acids of the oleic acid series as the impregnating nedium ,and the ,c a'ri-l ri e; for the dye substance, the tendency to crystallifilssubstantially inhibited and the conduc tivity of the mark made remains substantially fixed under all conditions of operation.

When acids of the oleic series are used for lead impregnation, I have moreover discovered that the electrical resistance of the mark is greatly reduced, to impart thereto conductivity so high that the statistical machines of the type referred to, will respond to the marking and effect the desired record thereof.

It is as the result of extensive experiments that I have found that the desired results are accomplished by resort to acids of the oleic series, and by these alone. No attempt will be made to set forth any theory to account for this sharp contrast in the action of one group of oleaginous materials, as contrasted with the apparently diametrically opposite action of any of the other common waxes and oils used for impregnation of leads.

While the oleic acid of commerce is reasonably satisfactory for the purpose, chemically pure oleic acid is to be preferred, since by its use variations in the electrical resistance of the mark from the standard desired, are avoided.

While oleic acid is ordinarily preferred, other acids of the oleic series may, if desired, be used to advantage. Among such other acids are palmitoleic acid, rapic acid and isooleic acid.

Desirably the porous conventional lead structure, (a fired mix of graphite and clay) is impregnated in a dye of rhodamine dissolved in alcohol usually 1 per cent by weight for a couple of hours, and then dried. Thereupon the leads are simply immersed in the oleic acid,

two hours and after removal from the tank, the leads may be drained and the surface dried with sawdust according to conventional practice. The dye will have become dispersed o'r dissolved in tha.,oleia-aEiHTTai5; ffsiilt ofitliis' oprfitio'fif More especially with grades of leads that are so porous as to carry a-relatively large proportion of the impregnating medium of oleic acid or the like, the electrical conductivity may become so high as to cause the sensitive electrical statistical machine to record even a small inadvertent smudge from the intended marking into an adjoining space or an inadvertent accidental mark in a space other than that intentionally marked. To guard against such excessive conductivity of the mark, the resistance thereof may be increased by admixing with the oleic acid other impregnating wax medium, preferably a hard wax, desirably spermacetti in appropriate proportions. For instance, the use of a mixture of one part of oleic acid to three parts of spermacetti as the impregnating medium will cause an increase in the resistance of the mark of 300 per cent. Depending on resistance specifications required, a lesser or a greater proportion of spermacetti may be used to effect respectively a lesser or a greater increase in resistance.

For the purpose of that statistical machine which records in response to the electrical resistance of the mark and is put out by the International Business Machine Corporation, it is ordinarily desirable to use an impregnating medium in proportions of one part by weight of oleic acid to three parts of spermacetti. The mark made by a lead thus impregnated when bridged by five copper bars each of width of .001" and pressed thereagainst with a pressure of three pounds, as used in such machine will show the desired electrical resistance of between 400 and 600 ohms.

The lead when impregnated as set forth is quite as effective from the standpoint of the type of mark made thereby as is a lead impregnated with conventional waxes and oils, and none of the other advantages of a lead of corresponding quality are sacrificed according to the present invention in attaining the specialized additional utilities set forth. With the dye incorporated in the leads as set forth, the black mark is indistinguishable to the eye from that made by ordinary lead pencils of corresponding grade and quality. The mark would be tested to see whether it has been made with the specialized lead by applying a drop of alcohol, which would immediately dissolve some of the rhodamine and leave a halo about the mark, as the drop spreads, giving the telltale indication which is lacking when alcohol is applied to a mark made by an ordinary pencil.

It will be understood that while the preferred pencil lead incorporates the indicator by which the identity of the lead can be readily determined, the invention in its broader aspects embraces within its scope a pencil lead devoid of such indicator. Such lead impregnated with acid of the oleic series will yield a mark of high electric conductivity, which will not vary materially even in a dry hot climate, and accordingly is thoroughly dependable. The conductivity will be calibrated as previously set forth by the addition of suitable amounts of hard wax such as spermacetti.

As many changes could be made in the above article, and many apparently widely different embodiments of this invention could be made without departing from the scope of the claims, it is intended that all matter contained in the above description shall be interpreted as illustrative and not in a limiting sense.

Having thus described my invention, what I claim as new and desire to secure by Letters Patent is:

1. A pencil lead presenting a mus structure impregnated with a fatty acid of the oleic acid series, and the mark of which has a predetermined ulgelu'aal. resis ance.

2. A penc lead comprising a fired mix of graphite and clay impregnated with a fatty acid of the oleic acid series, and the mark made by which has a low electrical resistance between predetermined limits.

3. A pencil lead of a fired mix of graphite and clay impregnated with a fatty acid of the oleic acid series, and with a soluble colored ingredient ineffective to modify the black color of the mark made by the lead.

4. A pencil lead of a fired mix of graphite and clay impregnated with oleic acid.

5. A pencil lead of a fired mix of graphite and clay impregnated with chemically pure oleic acid.

6. A pencil lead of a fired mix of graphite and clay impregnated with rapic acid.

7. A pencil lead of a fired mix of graphite and clay impregnated with isooleic acid.

8. A pencil lead of a fired mix of graphite and clay, the mark of which has high electrical conductivity above a predetermined minimum, said lead being impregnated with a mixture of fatty acid of the oleic acid series and spermacetti.

9. The lead claimed in claim 8 in which one part by weight of oleic acid is mixed with three parts of spermacetti.

10. A pencil lead including a porous structure of pigment and binder, having the pores thereof impregnated with a fatty acid of the oleic acid series and a dye.

11. A pencil lead including a porous structure of graphite and clay having the pores thereof impregnated with oleic acid carrying in solution or suspension therein a soluble indicating dye.

12. The process of preparing a pencil lead which consists in impregnating the structure with an alcohol solution of dye, drying the structure and then impregnating the same with a fatty acid of the oleic acid series.

13. The method of preparing a pencil lead which consists in impregnating the same in an alcohol solution of rhodamine dye, drying the lead and thereupon impregnating the same in oleic acid for a period of about two hours at a temperature of about C.

ISIDOR CHESLER 

